I've never done a mars bar but I have deep-fried ice cream. For ice cream you roll a ball if ice cream in crushed cereal (Special K works well) and then freeze it HARD (-10F) so you can drop it in hot, (400F) clean, oil and get a crisp coating before the stuff melts. Ten to fifteen seconds max. drizzle with caramel, garnish with whipped cream, serve immediately.
When you batter coat something, the batter doesn't adhere well if it is smooth and frozen (like fish filets) so a trick is to thaw slightly, dredge the item in seasoned flour and re-freeze so the item has textured surface for the batter to stick to. For a mars bar I'd season the flour with a bit of cinnamon and sugar if anything at all. Dust the bar and freeze (0F is probably fine for something like chocolate that's essentially solid at room temp) Shake off any excess, and dip it in batter. Any kind will do. Tempura might be nice and light.
I fry in a commercial deep-fryer with the flame isolated and a fire extinguishing system all primed and ready, I assume you don't have that, so be careful.
Peanut oil is good but it's expensive, especially outside the US. It gets very hot without smoking and doesn't alter the flavor of the food much. Fill your pan less than half full, wear heavy leather gloves and use a frybasket. I hope I don't need to tell a Brit how to fry things, I think you invented it. No need to fry it through all you want is the coating to cook. It'll be tasty right away, but it won't keep more than a few minutes before getting pretty nasty.